1. Field of the Invention
The present invention relates to a microquenched superconductor valve device and, more particularly, to an improved valve device having a bilayer ferromagnetic film.
2. Description of the Related Art
A magnetoquenched superconducting valve is a superconductor switching device with applications in digital and analog superconducting electronics. As explained in more detail below, locally strong fringe fields from a ferromagnetic film microfabricated on top of a superconducting strip create a weak link in the superconductor and suppress the critical current (Ic). The magnitude of the magnetic fringe field is controlled by manipulating the orientation of the ferromagnetic film and in this way the weak link can be created or removed, i.e., the device can be turned xe2x80x9conxe2x80x9d and xe2x80x9coff.xe2x80x9d The intrinsic remanence of the magnetization provides a natural memory function, and the device can be used as a nonvolatile storage cell. Current pulses in contiguous write wires can be used to control the magnetization orientation of the ferromagnetic and thereby control the switching state of the device. The magnitude of the control current can be made smaller than the critical current of the superconductor and thus the device can have current gain and can be used as a current amplifier.
The basic operation and the fundamental capabilities of the magnetoquenched semiconductor valve are described, for example, in U.S. Pat. No. 5,930,165 (Johnson et al), which is hereby incorporated by reference. However, to briefly summarize this description, reference will be made to FIG. 1 which shows an exploded perspective view of such a valve device, which is generally denoted 10. A superconducting bridge 10 is fabricated on a substrate 11 so as to extend parallel to the y axis and has a width Ws and critical current Ic(t), where t is the reduced temperature T/Tc and Tc is the critical temperature. A supply current is indicated by arrow 13. A thin ferromagnetic film 14 is fabricated on top of semiconductor 12 and is isolated therefrom by a very thin insulating layer 16, such as a tunnel barrier. Orthogonal write wires 17 and 19 provide first and second control currents indicated by arrows 18 and 20 and are formed by a conductor 22 and corresponding insulator 24. Ferromagnetic film 14 has two anisotropy axes such that the magnetization {right arrow over (M)} is stable along either the x or y axis.
The device of FIG. 1 works in the following way. When the magnetization is oriented along the x axis, dipolar fringe fields originating from the ends of film 14 are substantially parallel to the plane of superconductor 12 and have a weak magnitude in the vicinity thereof. Thus there is little or no effect on the superconducting properties of superconductor 12. This is called the unquenched state of superconductor 12, equivalently the xe2x80x9coffxe2x80x9d state of the device. However, when the magnetization of film 14 is oriented along the y axis, the dipolar magnetic fringe fields {right arrow over (B)} that originate at the edge of film 14 have a large magnitude in the vicinity of superconductor 12. In particular, there is a large component Bz perpendicular to the plane of superconductor 12. Much of the fringe field is shielded by Meissner currents in the superconductor 12, and most of the field is deflected to the side of the superconductor. However, the component Bz is sufficiently large that it exceeds the critical field in a small, local region directly beneath the edge of film 14 and a xe2x80x9cweak linkxe2x80x9d of normal metal is thereby created. The weak link causes substantial suppression of the critical current Ic(t) of the superconducting bridge 12. This is called the quenched state of the superconductor 12, equivalently the xe2x80x9conxe2x80x9d state of the switch.
The operation of the switch 10 relies on the remanent magnetization Mr of the ferromagnetic film 14, and therefore is intrinsically both xe2x80x9clatchingxe2x80x9d and xe2x80x9cnonvolatile.xe2x80x9d Energy is only used to set or reset the magnetization state of ferromagnetic film 14, and the quiescent power is zero. An upper bound of the switching speed of the device is the order of 1 nsec, and it may switch as rapidly as 1 psec. Integrated device operation invokes the application of current pulses to integrated write wires 17 and 19 shown in FIG. 1. A current pulse of appropriate amplitude and duration applied to the write wire 19 oriented parallel to the y axis orients the magnetization along the x axis, and the device is put into the unquenched or xe2x80x9coffxe2x80x9d state. A current pulse of appropriate amplitude and duration applied to the write wire 17 oriented parallel to the x axis orients the magnetization along the y axis, and the device is put into the quenched or xe2x80x9conxe2x80x9d state.
According to the invention, a magnetoquenched superconducting valve device is provided which provides important advantages over the basic valve device described above. Generally speaking, whereas the basic device uses a single ferromagnetic film or layer (corresponding to layer 14 of FIG. 1) and two magnetic anisotropy axes to provide fringe fields, the valve device of the invention uses a bilayer of ferromagnetic films as the ferromagnetic element that provides the fringe magnetic field. With this construction, a single magnetic anisotropy axis can be employed and this provides several associated advantages. More particularly, the areal dimensions of the device all can be reduced or shrunk, thereby permitting greater packing densities. Further, a single write wire can be used to control the magnetization orientation, thereby resulting in a simpler cell architecture.
An object of the invention is to provide an improved magnetoquenched superconductor valve device characterized by a single magnetic anisotropy axis. As indicated above, such a device needs only a single integrated write wire to control the device state. In contrast, the original or basic device requires two perpendicular write wires to control the device state. The write wire of the modified device of the invention can be fabricated from a superconducting material, thereby affording low power operation.
A further object of the invention is to provide an improved valve device with a single magnetic anisotropy axis which can be used as an integrated memory cell and which can be fabricated in a two dimensional array of cells, using a standard half-select process for addressing individual cells.
Yet another object of the invention is to provide an improved valve device with a single magnetic anisotropy axis which can be fabricated with smaller dimensions, and which can, therefore, be integrated with higher packing densities.
A further object of the invention is to provide an improved valve device that includes a superconducting ground plane which acts to increase the write fields (thereby decreasing the write current for a given, required write field), and to decrease the effects of the self-fields of the superconducting current in the bridge.
An additional object of the invention is to provide a modified ferromagnetic element comprising two ferromagnetic layers that can be incorporated in Hybrid Hall Effect (HHE) memory cells, such that each of the two ferromagnetic layers in the element stores a single bit of information. In this way, each HHE cell can store two bits of data. This object can, in principle, be generalized to n ferromagnetic layers storing n bits of information per cell.
In accordance with the invention, a magnetoquenched superconductor is provided which comprises: a substrate; a superconductor element disposed on the substrate; a first ferromagnetic film disposed on the superconductor element; and a second ferromagnetic film disposed on the first ferromagnetic film, said first and second films being magnetically coupled and having magnetizations which are switchable between a parallel relation wherein the films produce additive magnetic fringe fields that form a weak link in a portion of the superconductor element and an antiparallel relation wherein one of the films substantially absorbs the fringe field of the other film and the resultant fringe fields in the vicinity of the superconductor are such that a weak link is not formed in the superconductor.
Preferably, the device further comprises an insulating layer on the superconductor element for isolating the first ferromagnetic layer from the superconductor element. Advantageously, the device further comprises a separator layer disposed between the first and second ferromagnetic films. In one preferred embodiment, the separator layer comprises an insulator. In an alternative preferred embodiment, the separator layer comprises a nonmagnetic metal.
Preferably, the ferromagnetic films have different coercivities. Alternatively, or in addition, the ferromagnetic films have different magnetic moments.
In a presently preferred implementation, the ferromagnetic films have both different anisotropies and coercivities.
In one particular advantageous embodiment, one of the ferromagnetic films has a coercivity that is substantially higher than that of the other film and has a fixed orientation, and the other film has an orientation which is controlled.
Preferably, the superconductor element comprises a superconductor bridge and the device further comprises a superconductor ground plane.
In accordance with a further aspect of the invention, a random access memory circuit is provided which comprises: a plurality of magnetoquenched superconductor devices arranged in an array of rows and columns, a row of write lines each individually associated with a corresponding row of superconductor devices, and a column of write lines each individually associated with a corresponding column of super conductor devices; said superconductor devices each comprising: a substrate; a superconductor element disposed on said substrate; a first ferromagnetic film disposed on said superconductor element; and a second ferromagnetic film disposed on said first ferromagnetic film, said first and second films being magnetically coupled and having magnetizations which are switchable between a parallel relation wherein the films produce additive magnetic fringe fields that form a weak link in a portion of the superconductor element and an antiparallel relation wherein one of the films substantially absorbs the fringe field of the other film and the resultant fringe fields in the vicinity of the superconductor are such that a weak link is not formed in the superconductor; and said write lines controlling switching of individual ones of said superconductor devices.
In accordance with another aspect of the invention, a memory circuit is provided which enables nondestructive readout of data stored therein, the memory circuit comprising: a plurality of magnetoquenched superconductor devices arranged in an array of columns and rows; a row of bias lines each individually associated with a corresponding row of superconductor devices; a row of readout lines each individually associated with a corresponding column of superconductor devices; a plurality of sense amplifiers each connected to one of said readout lines; first means connected to each of said bias lines and to each of said superconductor devices for establishing a bias threshold for each of said superconductor devices; and second means connected to each of said sense lines and to each of said superconductor devices for isolating the respective superconductor devices from the corresponding sense line; said superconductor devices each comprising: a substrate; a superconductor element disposed on the substrate; a first ferromagnetic film disposed on the superconductor element; and a second ferromagnetic film disposed on said first ferromagnetic film, said first and second films being magnetically coupled and having magnetizations which are switchable between a parallel relation wherein the films produce additive magnetic fringe fields that form a weak link in a portion of the superconductor element and an antiparallel relation wherein one of the films substantially absorbs the fringe field of the other film and the resultant fringe fields in the vicinity of the superconductor are such that a weak link is not formed in the superconductor; and said bias lines controlling switching of the magnetizations of said superconductor devices and said sense lines sensing switching of the magnetization of said superconductor devices.
Preferably, the first and second means comprise first and second diodes, respectively, each of said first diodes being connected between an associated bias line and a corresponding superconductor device and each of said second diodes being connected between an associated sense line and a corresponding superconductor device.
In accordance with still another aspect of the invention, a random access memory circuit is provided which enables nondestructive readout of data stored therein, said circuit comprising: a plurality of magnetoquenched superconductor devices arranged in a two dimensional array of rows and columns; a plurality of columns of read/write lines each individually associated with a corresponding column of superconductor devices; a plurality of rows of read/write lines each individually associated with a corresponding row of superconductor devices; a sense line connecting all of said superconductor devices in series for receiving a bias current and for enabling readout of the presence of a voltage or lack thereof, said superconductor devices each comprising: a substrate; a superconductor element disposed on said substrate; a first ferromagnetic film disposed on said superconductor element; and a second ferromagnetic film disposed on said first ferromagnetic film, said first and second films being magnetically coupled and having magnetizations which are switchable between a parallel relation wherein the films produce additive magnetic fringe fields that form a weak link in a portion of the superconductor element and an antiparallel relation wherein one of the films substantially absorbs the fringe field of the other film and the resultant fringe fields in the vicinity of the superconductor are such that a weak link is not formed in the superconductor; and the read/write lines controlling switching of the magnetizations of said devices and the presence or absence of a said voltage being a function of whether a said weak link has been formed.
In accordance with yet another aspect of the invention, a hybrid Hall effect memory cell is provided, the cell comprising: a Hall plate; a first ferromagnetic layer disposed on said Hall plate; a first separating layer for electrically isolating the first ferromagnetic layer from said Hall plate; at least a second ferromagnetic layer disposed on first ferromagnetic layer; and a second separating layer electrically isolating said first and second ferromagnetic layers, each of said first and second ferromagnetic layers storing a single bit of data such that said memory cell stores at least two bits of data.
Further objects, features and advantages of the present invention will be set forth in, or apparent from, the detailed description of preferred embodiments thereof which follows.